Motion detected but no alert

I noticed that overnight I received a motion alert every 30 minutes, almost like clockwork but there was no actual movement (probably to do with air flow/air temp etc).

This morning, I went in and out once and there was no alert, and then I just happened to notice my pet walking through with no alert. Both times on the playback (i have SD card installed), it shows the green box that motion was detected, and it also follows the movement. But I received no alert on my phone. My alert setting is set to All Day.

TLDR: alert every 30 mins from 12am - 8am despite no movement in playback. Between 8am and noon, no alerts but confirmed movement and tracking.

What am I doing wrong?